Current state of the art electrical energy surge suppressors, such as AC outlet-mounted devices and stand alone devices, to which the power supply leads of one or more electrical appliances or electronic equipments are connectable, customarily include one or more interconnected passive, series impedances, such as inductors, and one or more non-linear voltage-limiting elements, such as gas discharge tubes, metal oxide varistors (MOVs), and solid state devices, such as silicon avalanche diodes (SADs), which serve to attenuate or clip the surge at a prescribed threshold level and limit the extent to which the magnitude of the surge exceeds the rated line voltage.
Examples of conventional surge suppressors are illustrated diagrammatically in FIGS. 1, 2 and 3, each having a pair of input terminals 11, 12 (line and neutral, respectively) across which the line voltage VL (e.g. 110 VAC) is applied, and a pair of output terminals 21, 22, to which the power supply leads of the equipment to be protected are connected. Coupled in circuit with either one or both of the input and output terminal pairs is one or a pair of series impedance elements, such as an inductor 31 coupled in series with terminals 11 and 21 in the devices of FIGS. 1 and 3, or respective series inductors 31, 32, series-coupled between terminals 11, 21 and 12, 22 in the device of FIG. 2. Connected across the line and neutral or ground, namely between each of the respective input and output terminal pairs 11, 12 and 21, 22 are non-linear voltage-limiting devices of the type referenced above. In the surge suppressor circuits of FIGS. 1 and 2, voltage-limiting elements 51 and 52 may comprise MOVs or SADs. In the configuration of FIG. 3, a gas discharge tube 53 is additionally coupled in series with element 51 across input terminals 11-12.
In response to the application of a line voltage transient (characteristically shown at 61) across input voltage terminals 11 and 12, the non-linear voltage limiting elements operate to limit or clamp the input voltage in accordance with their rated capability, while the series inductance(s) resist the flow of current to the secondary elements 51, 52 so as to reduce the surge from a typical peak value on the order of 6 kV to a voltage level on the order of 280-330 volts, depending upon the circuit employed, as shown by the suppressed surge characteristic 62.
In the circuit of FIG. 3, for example, gas discharge tube 53 turns on when the surge voltage exceeds a value on the order of 500-600 volts. The voltage across turned-on gas discharge tube 53 drops to a value on the order of 20 volts and places clamping MOV element 51, typically rated at 95 volts, in series with the tube. With the absorbing action of inductor 31 and clamping element 52, the let-through voltage appearing across output terminals 21-22 can be reduced to a value on the order of 280 volts, which may still be larger than that tolerated by a variety of electronic systems.